1. Field Of The Invention
This invention relates to an optical fiber having an aspherical lens at the tip thereof and to a method of making the same.
2. Description Of The Prior Art
An optical communication system includes a source of light energy and an associated receiver connected over an optical fiber waveguide. The measure of the power coupled from the source into the fiber or from the fiber to the receiver is termed coupling efficiency.
The typical optical fiber has a core approximately nine (9) micrometers in diameter, the core being formed of a material that exhibits a first predetermined index of refraction. The core is surrounded by an outer layer of a cladding material that exhibits a second predetermined index of refraction. The overall outer diameter of the typical optical fiber is on the order of one hundred twenty five (125) micrometers.
The optical fiber usually has a lensed end at its tip. The lensed end is typically spherical, although it is known that the lensed end may be aspherical in shape.
Although a spherical lens is easy to produce and is generally sufficient to the meet the needs of the system in which it is placed, a spherical lens is subject to spherical aberration. Such spherical aberration lowers coupling efficiency and thus renders such a fiber less preferred for low loss, high gain uses, such as laser optical amplifiers.
It has been recognized that an aspherical lens reduces spherical aberration and improves coupling efficiency for any sized lens. However, an aspherical lens does not appear to be widely used on optical fiber waveguides, perhaps because of a perceived difficulty in manufacturing the same.
U.S. Pat. No. 4,565,558 (Keil et al.) and U.S. Pat. No. 4,589,897 (Mathyssek et al.) both relate to the formation of a spherical or aspherical lensed end on an optical fiber. The apparatus disclosed in these patents utilizes two clamps, at least one of which moves relatively to the other while an electric arc heats a portion of the fiber between the clamps. As a constriction appears as the result of constant tension and heat, the tension is dropped and a further constriction occurs leading to a separation which solidifies when the heat is cut off to form a lens on a tapered fiber.
Blaudau and Rossberg, Journal of Lightwave Technology, Vol. LT-3, No. 3, April 1985 teach making an aspherical lens by first forming a bulbous spherical lens on a fiber and then welding a cylinder of pure quartz at the center of the bulb. Upon remelting the pure quartz flows out to form an aspherical surface.
U.S. Pat. Nos. 4,243,349 and 4,370,021 (both to Khoe et al.) teach flattening the end of an optical fiber to produce a semi-elipsoidal lens.